1. Field of the Invention
This invention relates to an electroless Ni—B plating liquid, an electronic device and a method for manufacturing the same. More particularly, this invention relates to an electroless Ni—B plating liquid useful for forming a protective film for protecting the surface of the interconnects of an electronic device which has such an embedded interconnect structure that an electric conductor, such as silver or copper, is embedded in fine recesses for interconnects formed in the surface of a substrate such as a semiconductor substrate, and to an electronic device having the interconnects-protecting film formed by using the plating liquid, and a method for manufacturing the same.
2. Description of the Related Art
As a process for forming interconnects in an electronic device, the so-called “damascene process” which comprises filling trenches for interconnects and contact holes with a metal (electric conductor), is coming into practical use. According to this process, aluminum or, more recently a metal such as silver or copper, is filled into trenches for interconnects and contact holes previously formed in the interlevel dielectric of a semiconductor substrate. Thereafter, an extra metal is removed by chemical mechanical polishing (CMP) so as to flatten the surface of the substrate.
In the case of interconnects formed by such a process, the embedded interconnects have an exposed surface after the flattening processing. When an additional embedded interconnect structure is formed on such an exposed surface of the interconnects of a semiconductor substrate, the following problems may be encountered. For example, during the formation of a new SiO2 in the next interlevel dielectric forming process, the exposed surface of the pre-formed interconnects is likely to be oxidized. Further, upon etching of the SiO2 film for formation of via holes, the pre-formed interconnects exposed on the bottoms of the via holes can be contaminated with an etchant, a peeled resist, etc.
In order to avoid such problems, it has conventionally been performed to form a protective film of SiN or the like not only on the interconnect region of a semiconductor substrate where the interconnects are exposed, but on the whole surface of the substrate, thereby preventing the contamination of the exposed interconnects with an etchant, etc.
However, the provision of a protective film of SiN or the like on the whole surface of a semiconductor substrate, in an electronic device having an embedded interconnect structure, increases the dielectric constant of the interlevel dielectric, thus inducing delayed interconnection even when a low-resistance material such as silver or copper is employed as an interconnect material, whereby the performance of the electronic device may be impaired.
In views of this, it may be considered to selectively cover the surface of the exposed interconnects with a Ni—B alloy film having a good adhesion to an interconnect material such as silver or copper and having a low resistivity (ρ). A plated Ni—B film, obtained by electroless Ni—B plating, is either a crystalline or an amorphous plated film depending on the boron content of the film. In this regard, a crystalline plated film is obtained when the boron content of the film is less than 10 at % (atomic %), and an amorphous plated film is obtained when the boron content of the film is 10 at % or more, generally.
When a plated Ni—B film is used for the purpose of protecting the interconnects of an electronic device having an embedded interconnect structure, the plated film is required to be thermally stable. From this point of view, it is necessary to use a crystalline plated film having a boron content of less than 10 at %. This is because a crystalline plated Ni—B film maintains its crystallinity after a heat treatment, whereas an amorphous Ni—B plated film forms a Ni—B compound upon the heat treatment and thus becomes an unstable film.
However, when an intended crystalline Ni—B film, for the purpose of protecting the interconnects of an electronic device having an embedded interconnect structure, is formed by electroless plating by using a plating liquid that is formulated to provide a plated film having a lowered boron content, the plating rate is likely to become too high to make a proper control of the process.
In this regard, in electroless plating, the reaction time is equal to the solid-liquid contact time between the plating liquid and an object to be plated. Further, a plated Ni—B film to be used for protecting the interconnects of an electronic device must be as thin as several tens to several hundreds nm. Accordingly, an enhanced plating rate makes the process control more difficult.